CEO of state-controlled Russian oil company Rosneft Igor Sechin commissions new equipment at the Rosneft oil refinery in the Black Sea port of Tuapse, southern Russia, in 2013. The U.S. Department of the Treasury on Monday sanctioned seven Russian government officials, including two key members of the Russian leadership’s inner circle, and 17 entities.

LONDON — U.S. sanctions on the president of Russia’s largest oil producer, Rosneft, have intensified concerns for foreign investors in the country, particularly BP, which has a 20 percent stake in the company and whose CEO — an American — sits on its board.

Though BP says it remains committed to Rosneft, analysts say the worry is growing that the sanctions by the U.S. were merely the prelude to a tougher one against the company as a whole. That could push BP to reconsider its investment and threaten other foreign activities in Russia, such as ExxonMobil’s deal to explore the Arctic for oil.

Targeting Rosneft President Igor Sechin is seen as a warning shot of sorts — singling out a confidante of President Vladimir Putin to underscore that the West will next time go after Russia’s biggest companies if Moscow doesn’t help to resolve the crisis in Ukraine. That would make it illegal for U.S. investors to deal with the likes of Rosneft.

The fact that such sanctions might have an impact on Western interests gives them more heft, said Philip Hanson, associate fellow in the Russia and Eurasia program at the think tank Chatham House.

“Sanctions are a message,” he said. “They are an instrument that are somewhere between pure diplomacy —talking — and warfare. If that message also has knock-on effects on you, it becomes more weighty.”

Currently, the sanctions on Sechin himself also complicate life for companies like BP. Its CEO, Bob Dudley, may be barred from communicating with Sechin, raising practical questions about how they will continue to work together. For now, Dudley plans to attend board meetings, like always.

“Rosneft itself has not been sanctioned,” Dudley told analysts Tuesday on a call that was intended to focus solely on quarterly earnings. “So we will continue to work in whatever the appropriate manner is with Rosneft, primarily as a shareholder but also as a partner.

“Of course, BP is not alone in being a big energy investor in Russia and a partner of Rosneft’s.”

Texas’ ExxonMobil has been investing in Russia for years, including a joint venture to explore for oil and gas in the Arctic. Eni of Italy and Statoil of Norway also have deals.

In the meantime, the sanctions and the Ukrainian crisis are already having a monetary impact on foreign companies, if indirectly. Fears of the cost of sanctions on Russia’s economy have caused the ruble to drop sharply in recent months, cutting into the value of earnings from the country.

BP said Tuesday that its earnings from its stake in Rosneft fell sharply in the first quarter because of the ruble’s drop.

The British company’s stake in Rosneft yielded a pre-tax replacement cost profit — an industry measure of earnings — of $271 million in the quarter ended March, down from $1.08 billion during the fourth quarter of last year.

Sechin, a former KGB officer like Putin, has been president of Rosneft since the early 1990s. He is seen as the mastermind behind the 2003 assault on the private oil company Yukos, whose founder, Mikhail Khodorkovsky, was jailed following disputes with the Kremlin. Rosneft seized Yukos’s most valuable assets, making it Russia’s largest company.

“They are targeting Igor Sechin, not the company itself — but it leaves open the question of whether (the U.S.) will target the company itself,” said Adnan Vatansever, a senior lecturer at the Russia Institute at King’s College London.